AU2014373596A1

AU2014373596A1 - Security device for security document

Info

Publication number: AU2014373596A1
Application number: AU2014373596A
Authority: AU
Inventors: Sylvain Chosson; Dieter Sauter
Original assignee: Orell Fuessli Sicherheitsdruck AG
Current assignee: Orell Fuessli Sicherheitsdruck AG
Priority date: 2013-12-23
Filing date: 2014-12-22
Publication date: 2016-05-26
Anticipated expiration: 2034-12-22
Also published as: WO2015095975A1; CA2934735A1; RU2016130280A; RU2671039C2; AU2014373596B2; WO2015095977A1; EP3089878A1; US20160321852A1; US10255743B2; MX2016006760A; RU2016130280A3

Abstract

A security device (1) for verifying an authenticity of a security document (100) comprises an at least partially transparent substrate (2) with a first surface (3) and a second surface (4). A first pattern (10) is arranged on the first surface (3). A second pattern (20) is arranged on said second surface (4). The first (10) and the second (20) pattern each comprise a plurality of pixels with at least three different gray levels visible from a macroscopic perspective. The first (10) and second (20) pattern cover only gray levels in a range between 20% black and 80% black, in particular between 35% black and 65% black. The first pattern (10) is inverted with respect to the second pattern (20). As an effect a transmission-mode-viewer (PI) sees a homogeneous image, if the first and the second pattern are superposed, because the first and the second pattern are indiscernible, cancel each other out in the transmission viewing mode. Information disappears. However, in a reflection viewing mode (P2, P3), information contained in the first or in the second pattern can be discerned since the first and the second pattern do not cancel each other out.

Description

WO 2015/095977 PCT/CH2014/000178 Security device for security document Technical Field 5 The invention relates to a security device for verifying an authenticity of a security document as well as to a security document, e.g., a banknote, a pass port, a document of value, a certificate, or a credit card which comprises such a security device. Furthermore, io the invention relates to a method for verifying the au thenticity of such a security document. Background Art is US 2006/0197990 Al discloses a superposition of two tally images, thus revealing a hidden image. The hidden image cannot be reconstructed from a single tally image. WO 97/47487 describes a security device hav 20 ing two simple patterns printed on opposite sides of a substrate, which generate different images when seen in reflection and transmission. EP 1580025 describes a security device which shows, under a certain viewing angle, optically recog 25 nizable features. Disclosure of the Invention It is an object of the present invention to 30 provide a security device for verifying an authenticity of a security document. Another object of the invention is to provide a security document comprising such a secu rity device. Another object of the invention is to pro vide a method for generating a security device. Yet an 35 other object of the invention is to provide a method for verifying the authenticity of such a security document.

WO 2015/095977 PCT/CH2014/000178 2 These objects are achieved by the devices and the method of the independent claims. Accordingly, a security device for verifying 5 an authenticity of a security document (such as a bank note, a passport, a document of value, a certificate, or a credit card) comprises an at least partially transpar ent substrate with a first surface and a second surface. Herein, the term "at least partially trans 10 parent" relates to an optical property of a nonzero transmission of light at at least one wavelength, in par ticular in the visible regime between 380 nm and 780 nm. Thus, in a transmission viewing mode, a nonzero amount of light can be shone through said substrate. Advantageous 15 ly, a transmittance of the substrate is higher than 50%, at least for one transmitted wavelength (which is in par ticular in the visible regime between 380 nm and 780 nm). Advantageously, the substrate is flat and/or flexible (e.g., its thickness is smaller than 500 pm, in 20 particular smaller than 120 pm) and the second surface can be on the opposite side of a flat substrate than the first surface. This simplifies the application in securi ty documents which are usually flat and/or flexible to some degree. 25 Furthermore, the security device comprises a first pattern (e.g., a halftone, grayscale, or a color image) which is arranged on said first surface of said substrate. Furthermore, the security device comprises a second pattern (e.g., a halftone, grayscale, or a color 30 image) which is arranged on said second surface of said substrate, e.g., opposite said first surface (see above). Furthermore the first pattern and the second pattern each comprise a plurality of pixels with at least three dif ferent gray levels visible from a macroscopic perspec 3s tive. The term "three different gray levels visible from a macroscopic perspective" relates to the fact that pat terns might be printed in halftone, which simulates con- WO 2015/095977 PCT/CH2014/000178 3 tinuous tone imagery through the use of dots or other symbols, varying either in size, in shape or in spacing, thus generating a gradient like effect. The term "three different gray levels visible from a macroscopic perspec 5 tive" means that the human eye would see different gray levels, although the dots in microscopic perspective have all the same grey level, in particular black. The first pattern and the second pattern cov er only gray levels in a range between 20% black and 80% 10 black, in particular between 35% black and 65% black, wherein the first pattern is inverted with respect to the second pattern. The restriction to this range yields a perceived black level in transmission viewing mode of the superposed inversed first and second patterns between 75% is and 84%, in particular between 75% and 77.25%. This is a range of black levels, in particular a range of black levels of 5%, where the black levels are not or at least hardly distinguishable or discernible by the naked eye of a viewer without visual aids. 20 The term "inverted" in particular means the generation of the contrary black level in the gray scale space, e.g. a 20% black level is inverted to an 80% black level. In more general terms, a black level of x% is in verted to a black level of (100% - x%). 25 The first pattern on the first surface is ad vantageously arranged fully in register with the second pattern on the second surface. As an effect, in particular in a range be tween 35% black and 65% black, a transmission-mode-viewer 30 (e.g., with a naked eye without visual aids) sees a homo geneous image, if the first and the second pattern are superposed, because the first and the second pattern can cel each other out in the transmission viewing mode. However, in a reflection viewing mode, for at 35 least one reflected wavelength (which is advantageously the same wavelength than the transmitted wavelength dis cussed above) from said security device, information con- WO 2015/095977 PCT/CH2014/000178 4 tained in the first or in the second pattern can be dis cerned since the first and the second pattern do not can cel each other out. As an effect, according to the invention, the 5 visual appearance and reconstructable information content of the security device depends on the viewing mode and security is thus enhanced considerably. Advantageously the average black level of the 10 first and second pattern is 50% +/- 5%. Thus the range of perceived black level in transmission viewing mode of the superposed inversed first and second pattern is minimized for the corresponding range of black levels on the first and the second pattern. 15 Advantageously the first and the second pat tern are applied, in particular printed, by absorbing inks. This increases the contrast between the absorbing ink and the substrate in the reflection viewing mode. 20 Advantageously, the substrate comprises mul tiple layers with the same or different optical proper ties (such as transmission spectra). Thus, more specific effects can be realized and security is enhanced. 25 In particular, the first and/or the second pattern can be covered with one or more additional lay er(s), e.g., for reducing specular reflections from the first and/or second substrate surface. 30 In an advantageous embodiment of the security device, the first pattern is applied, in particular printed (e.g., via offset printing, screen printing, or sublimation printing), onto said first surface of said 35 substrate and/or the second pattern is applied, in par ticular printed (e.g., via offset printing or screen printing, or sublimation printing), onto said second sur- WO 2015/095977 PCT/CH2014/000178 5 face of said substrate. Thus, the security device can be manufactured more easily. Optionally, a primer layer can be applied be low the first and/or second pattern in order to ensure 5 the stability of the printed inks. In another advantageous embodiment of the se curity device, the first region of the first pattern has an inverted transmittance and an inverted reflectivity 10 with respect to the third region of said second pattern. Furthermore, in this embodiment, the second region of the first pattern has an inverted transmittance and an in verted reflectivity with respect to the fourth region of said second pattern. is Herein, the terms "inverted transmittance" and "inverted reflectivity" relate to a transmit tance/reflectivity value (e.g., of a specific region) which is "inverted" with respect to an ideal 100% trans mission/reflection at one or more wavelength(s) (in par 20 ticular in the visible regime between 380 nm and 780 nm) and with respect to another transmittance/reflectivity value (e.g., that of another region). As examples, for a 90% transmittance of the first region, an inverted trans mittance of the third region would be 10%. A 20% reflec 25 tivity of the third region is inverted with respect to an 80% reflectivity of the first region. Thus, it is easier to select the transmit tances and reflectivities of the first to fourth regions such that the above-discussed visual appearance effects 30 occur in the transmission and reflection viewing modes. More advantageously, the whole first and sec ond patterns (i.e., all regions in the respective pat terns) have inverted transmittances and reflectivities with respect to each other. 35 In another advantageous embodiment of the se curity device, the gray levels of the first pattern (10) WO 2015/095977 PCT/CH2014/000178 6 and of the second pattern (20) are indiscernible at least when an overall (i.e., spatially integrated over the whole security device) transmitted light intensity through the security device (in the transmission viewing 5 mode) outshines an overall (i.e., spatially integrated over the whole security device) reflected light intensity from the security device at least by a factor of 5. In other words, in this embodiment, a definition for "trans mission viewing mode" is that the overall transmitted io light intensity through the security device outshines an overall reflected light intensity from the security de vice at least by the above-mentioned factor. Thus, it is easier to select the transmit tances and reflectivities of the first to fourth regions 15 such that the above-discussed visual appearance effects occur in the transmission viewing mode. In another advantageous embodiment of the se curity device, the gray levels of the first pattern (10) 20 and of the second pattern (20) are discernible at least when an overall (i.e., spatially integrated over the whole security device) reflected light intensity from the security device outshines an overall (i.e., spatially in tegrated over the whole security device) transmitted 25 light intensity through said security device at least by a factor of 5. In other words, in this embodiment, a def inition for "reflection viewing mode" is that the overall reflected light intensity from the security device out shines an overall transmitted light intensity through the 30 security device at least by the above-mentioned factor. Thus, it is easier to select the transmit tances and reflectivities of the first to fourth regions such that the above-discussed visual appearance effects occur in the reflection viewing mode. 35 In another advantageous embodiment, the secu rity device further comprises a third pattern (e.g., a WO 2015/095977 PCT/CH2014/000178 halftone, grayscale, or a color image) with different gray levels arranged on or in said substrate (or, in case of a multilayered substrate, e.g., between different sub strate layers). In said transmission viewing mode and in 5 said reflection viewing mode, said different gray levels of said third pattern (30) are discernible. As an effect, a transmission-mode-viewer as well as a reflection-mode-viewer can reconstruct infor mation contained in the third pattern (i.e., the 10 fifth/sixth regions). This is possible in the transmis sion as well as in the reflection viewing modes. In another advantageous embodiment of the se 15 curity device, the first pattern and/or the second pat tern and/or the substrate comprises a color filter. This makes it easier to select one or more transmitted and/or reflected wavelength(s). 20 As another aspect of the invention, a securi ty document (e.g., a banknote, a passport, a document of value, a certificate, or a credit card) comprises a secu rity device as described above. The security device is 25 advantageously arranged in a window (i.e., a transparent region) of (the substrate of) the security document. As an effect, the visual appearance and reconstructable in formation content of the security document can be more easily made dependent on the viewing mode. Thus, security 30 is enhanced and counterfeiting is considerably aggravat ed. Advantageously, such a security document fur ther comprises a light absorber, in particular arranged as at a distance to the security device. Then, for example by folding the security document along an applied, in particular printed, folding line, the light absorber can WO 2015/095977 PCT/CH2014/000178 8 be brought into overlap with the security device. As an effect, the amount of transmitted light is reduced by the light absorber and thus a reflection viewing mode is reached more easily. As an effect, handling is improved 5 when the authenticity of the security document is to be checked. Advantageously, the light absorber has a re flectivity of less than 50% at least for said at least 10 one reflected wavelength from said security device and/or the light absorber has a transmittance of less than 50% at least for said at least one transmitted wavelength through said security device. The light absorber can, e.g., comprise a region of the security document which is 15 covered by a dark color, e.g., 100 % black. As an effect, the reflection viewing mode of the security device is reached more easily and handling is improved when the au thenticity of the security document is to be checked. 20 As another aspect of the invention, a method for verifying an authenticity of a security document as described comprises steps of - providing the security document which com prises a security device as described above, 25 - from a first viewing position acquiring a first image of said security device in a transmission viewing mode (e.g., against a ceiling lamp), - from a second viewing position (which can be the same or a different position than the first view 30 ing position) acquiring a second image of said security device in a reflection viewing mode. Hereby, the first pattern is oriented towards the second viewing position. Furthermore, the method comprises a step of - deriving said authenticity of said security 35 document using the first (transmission viewing mode) im age and using the second (reflection viewing mode) image.

WO 2015/095977 PCT/CH2014/000178 9 Because of the specific and different visual appearances in transmission viewing mode (first region cannot be discerned from third region and/or second re gion cannot be discerned from fourth region) and reflec 5 tion viewing mode (first region can be discerned from third region), the authenticity of the security document is easier to derive. Advantageously, during the step of acquiring io said second image, an overall reflected light intensity from said security device outshines an overall transmit ted light intensity through said security device at least by a factor of 5. Thus, the reflection viewing mode is easier to establish. 15 In another advantageous embodiment, during said step of acquiring said first image, an overall transmitted light intensity through said security device outshines an overall reflected light intensity from said 20 security device at least by a factor of 5. Thus, the transmission viewing mode is easier to establish. Advantageously, the method comprises a step of bringing a light absorbing device into an overlap with 25 said security device. Thus, an amount of transmitted light through the security device is reduced and the re flection viewing mode is easier to establish. Then, the step of acquiring said second image of said security de vice is carried out with said light absorbing device be 30 ing arranged in said overlap with said security device, e.g., opposite said second viewing position. This simpli fies the handling of the security document for acquiring the reflection viewing mode image. 35 Advantageously, from a third viewing position (which is, again, the same or different from the first and/or the second viewing position(s)), a third image of WO 2015/095977 PCT/CH2014/000178 10 said security device is acquired in a reflection viewing mode, but now with said second pattern being oriented to wards said third viewing position. Then, this third image is also used in said step of deriving said authenticity 5 of said security document. Thus, the security can be fur ther enhanced. As another aspect of the invention, a method for generating a first pattern and a second pattern for 10 use in a security device as described above comprises steps of - providing an at least partially transparent substrate (2), in particular with specular reflection in a reflection viewing mode, and with a first surface (3) 15 and a second surface (4), - providing a first source image with at least three different gray levels, - modifying a contrast of said first source image to generate a first modified image, wherein the 20 first modified image covers only gray levels in a range between 20% black and 80% black, in particular 35% and 65% black. For example the first source image has black level between 0% and 100% black. The brightness and con trast of the first source image are modified to ensure 25 that all black level are between 20% and 80% black, in particular between 35% and 65% black. In other words, its histogram of gray levels is shrunken. It should be noted that modifying the contrast of said first source image is unnecessary, if it already meets the stated range re 30 quirements. Furthermore, the method comprises steps of - inverting the first modified image for yielding a second modified image, - generating a first pattern from said first 35 modified image, - generating a second pattern from said sec ond modified image, WO 2015/095977 PCT/CH2014/000178 il - applying, in particular printing, the first pattern (10) on said first surface (3) of said substrate (2), - applying, in particular printing, the sec 5 ond pattern (20) on said second surface (4) of said sub strate (2). The term "generating" can e.g. mean that the first pattern or second pattern are equal to the first modified pattern or to the second modified pattern re 10 spectively or it can e.g. mean that the first pattern or the second pattern are a result of mixing the first pat tern with a third pattern and/or a result of mixing the second pattern with a third pattern respectively. The first pattern on the first surface is ad 15 vantageously arranged fully in register with the second pattern on the second surface. Advantageously, for generating the first and or second pattern, a third pattern is mixed with the first modified 20 image to generate the first pattern and/or wherein the third source image is mixed with the second modified im age to generate the second pattern (20) such that in a transmission viewing mode only the third pattern (30) is visible while the first pattern (10) and the second pat 25 tern (20) cancel each other out because they are inverted images. The third pattern can be generated by shrinking the histogram of gray levels. Advantageously the average black level of the first and second pattern is 50% +/- 5%. Thus the range of 30 perceived black level in transmission viewing mode of the superposed inversed first and second pattern is minimized for the corresponding range of black levels on the first and the second pattern. 35 Advantageously the first and the second pat tern are applied, in particular printed, by absorbing WO 2015/095977 PCT/CH2014/000178 12 inks. This increases the contrast between the absorbing ink and the substrate in the reflection viewing mode. Advantageously the substrate is partially re 5 flecting, in particular specularly reflecting. The term "specularly reflecting" in particular relates to the mir ror-like reflection of light from a surface, in which light from a single incoming direction is reflected into a single outgoing direction. The direction of incoming 10 light and the direction of outgoing light reflected make the same angle with respect to the surface e normal. Remark: The invention is not limited to halftone or i5 grayscale patterns. Although the description and figures herein mainly focus on halftone and grayscale patterns for the sake of clarity, analogous considerations can be made for each color channel of color patterns which ren ders the subject-matter of the invention feasible for 20 color patterns. Hence, terms of the type "gray level", or "black" are understood to express the color density of the patterns. For example the term "gray level of x% black" expresses that the color density is x%. 25 This definition can be used for any kind of ink or dye, e.g. red, green or blue dyes. However, the invention shows strongest effects at a wavelength where the ink or dye is fully absorbing. In other words, for best results over the whole visible range, the patterns 30 are advantageously printed in black ink or dye, in which case the term "gray level of x% black" expresses a pat tern that absorbs x% over the whole visible spectrum. The described embodiments similarly pertain to the devices and the methods. Synergetic effects may 35 arise from different combinations of the embodiments alt hough they might not be described in detail.

WO 2015/095977 PCT/CH2014/000178 13 Brief Description of the Drawings 5 The invention will be better understood and objects other than those set forth above will become ap parent when consideration is given to the following de tailed description thereof. Such description makes refer ence to the annexed drawings, wherein: 10 Fig. 1 shows a first pattern 10 and a second pattern 20 for use in a security device 1 as well as a combination of this first pattern 10 with this second pattern 20 in a transmission viewing mode, Fig. 2 shows a security device 1 according to 15 a first embodiment of the invention, the security device 1 comprising a transparent substrate 2 and a first pat tern 10 and a second pattern 20 arranged on opposite sur faces 3, 4 of said substrate 2, Fig. 3 shows a security document 100 compris 20 ing a security device 1 according to a second embodiment of the invention, Fig. 4 shows a security device 1 according to a third embodiment of the invention, the security device 1 comprising a first pattern 10, a second inverted pat 25 tern 20, and a third pattern 30, Fig. 5 schematically shows a security docu ment 100 comprising the security device 1 of fig. 2, a light absorber 5, and a folding line 500, Fig. 6 schematically shows the security de 30 vice 1 of fig. 2 in a transmission viewing mode, Fig. 7 schematically shows the security de vice 1 of fig. 2 in a reflection viewing mode with specu lar reflection, and Fig. 8 schematically shows the security de 35 vice 1 of fig. 2 in a reflection viewing mode with specu lar reflection and second pattern attenuation by a light absorber 5.

WO 2015/095977 PCT/CH2014/000178 14 Fig. 9 schematically shows another security device in the transmission viewing mode with an illustra tion of the light intensity in different steps. Fig. 10 schematically shows another security 5 device in the reflection viewing mode with an illustra tion of the light intensity in different steps. Modes for Carrying Out the Invention 10 Fig. 1 shows a first pattern 10 and a second pattern 20 for use in a security device 1 according to the invention (the security device 1 is not shown here). In this figure, the first pattern 10 is a grayscale image with a gradient from 100% white (i.e., 0% black) to 100% is black. The second pattern 20 is an inverted pattern with regard to the first pattern 10, i.e., it is a grayscale image with a gradient from 100% black to 0% black. When the first pattern 10 is overlaid with the second pattern 20 (i.e., when a first region 11 fully 20 coincides with a third region 23 and a second region 12 fully coincides with fourth region 24) and viewed in a transmission viewing mode, a grayscale image 200 as de picted in the lower part of figure 1 is observed. Specif ically, a grayscale image going from 100 % black to 75 % 25 black back to 100 % black is yielded. The upper part of figure 1 shows the black levels of the single patterns 10 and 20 as well as of the combined grayscale image 200 (in transmission viewing mode) as functions of position. 30 What can be seen from the diagram is that in the transmission viewing mode (i.e., with transmissions through the first and through the second pattern being combined), the first region 11 is indiscernible from the second region 12 of the first pattern 10, because both as the first region 11 and the second region 12 show the same gray levels of 84% black (see the points labeled 12+24 and 11+23 of the curve labeled 200 in the diagram).

WO 2015/095977 PCT/CH2014/000178 15 Similarly, a third region 23 is indiscernible from a fourth region 24 of the second pattern 20, because both the third region 23 and the fourth region 24 show the same gray levels of 84% black (see the above-referenced 5 points). This is, because the first region 11 of the first pattern 10 fully coincides with the third region 23 of the second pattern 20 (see vertical line). Similarly, the second region 12 of the first pattern 10 fully coin 10 cides with the fourth region 24 of the second pattern (see vertical line). Furthermore, the first pattern 10 (i.e., all regions) is inverted with respect to the sec ond pattern 20. 15 One possible theoretical approach to explain this is the so-called Demichel equation. For 2 colors, the Demichel equation shows that for the superposition of a layer of color Cl with a density dl and of a layer of color C2 with a density d2 (both layers having a random 20 halftoning), a surface coverage of white w=(1-dl)x(1-d2), a perceived color Cl = dl x (1-d2), and a perceived color C2 = d2 x (1-dl). If both colors Cl and C2 are black and if 25 d2 = 1 - dl (inverted patterns), the density of black b (i.e., b = 1 - w) for the superposed image equals to b = 1 - dl + d1 2 . This corresponds to the curve labelled 200 in the diagram of figure 1. As an example, the first region 11 of the 30 first pattern 10 and the fourth region 24 of the second pattern 20 both are 80% black. The second region 12 of the first pattern 10 and the third region 23 of the sec ond pattern 20 both are 20% black. Hence, the first re gion 11 has a different transmittance and reflectivity 35 than the second region 12 and the third region 23 has a different transmittance and reflectivity than the fourth region 24. The superposition of the first region 11 with WO 2015/095977 PCT/CH2014/000178 16 the third region 23 yields b = 1 - 0.8 + 0.82, i.e., b = 84% black. This is the same value as for the superposi tion of the second region 12 with the fourth region 24, namely b = 1 - 0.2 + 0.22 = 84% black. Note that a 100% 5 transmittance of the substrate is assumed here (substrate not shown!). Thus, in a transmission viewing mode (i.e., in a superposition of the first pattern 10 with the sec ond pattern 20), the first region 11 is indiscernible 10 from the second region 12 and the third region 23 is in discernible from the fourth region 24. As can be further seen from the Demichel equation: * With the full range of grayscales (see 15 range 1), the perceived black level in transmission view ing mode of the superposed inversed images ranges between b = 100% and 75%. * With a smaller range of grayscales (see range 2) such as 0.2 to 0.8 (i.e., the example above), 20 the perceived black level of the superposed inversed im ages ranges between b = 84% and 75% (horizontal dashed lines). * With an even smaller range of grayscales (see range 3) such as 0.35 to 0.65, the perceived black 25 level of the superposed inversed images ranges between b = 77.25% and 75%. This is a range of black levels b where the black levels are not distinguishable or indis cernible by the naked eye of a viewer without visual aids. Thus, in this example, in a transmission viewing 30 mode through first pattern 10 and second pattern 20, a first region 11' would be indiscernible from a second re gion 12' and a third region 23' would be indiscernible from a fourth region 24'. In general, it can be stated that regions with transmitted light intensity-differences 3s below 5% cannot be discerned. If the first pattern 10 is viewed in a re flection viewing mode (e.g., with an overall reflected WO 2015/095977 PCT/CH2014/000178 17 light intensity from the first pattern 11 outshining an overall transmitted light intensity at least by a factor of 5), the full superposition of the first pattern 10 with the second pattern 20 does not take place any more 5 and the first region 11 is thus discernible from the sec ond region 12 due to their different reflectivities. In general, it can be stated that regions with reflected light intensity-differences above 5% can be discerned. Thus, very specific patterns can be created 10 under different viewing conditions and security in en hanced. Fig. 2 shows a security device 1 with a transparent flat flexible multilayer polymer substrate 2 15 with a thickness of 110 pm. A first pattern 10 (a gray scale image) is applied, in particular printed, onto a first surface 3 of the substrate 2 and a second pattern 20 (a grayscale image) is applied, in particular printed, onto a second opposite surface 4 of the substrate 2. The 20 first pattern 10 comprises a first region 11 ("OFS" and "123" in 80% black) and a second region 12 (background in 20% black) which does not overlap but is adjacent the first region 11. The second pattern 20 comprises a third region 23 ("OFS" and "123" in 20% black) and a fourth re 25 gion 24 (background in 80% black) which does not overlap but is adjacent to the third region 23. The first region 11 fully coincides with the third region 23 and the sec ond region 12 fully coincides with the fourth region 24. This is e.g. achieved by a high registration printing 30 process of the first and second patterns 10, 20 onto the first and second surfaces 3,4 of the polymer substrate 2. As shown for the first image Il taken from a first viewing position P1 in a transmission viewing mode, the first region 11 is indiscernible from the second re 35 gion 12, because the whole image appears at a uniform gray level of 84% black. As discussed above with regard to figure 1, other combinations of black levels would be WO 2015/095977 PCT/CH2014/000178 18 possible as well as long as the first/second regions 11,12 and the third/fourth regions 23,24 remain indis cernible in the transmission viewing mode. However, in a reflection viewing mode, which s is here facilitated by overlaying the security device 1 with a light absorber 5, the first region 11 is discerni ble from the second region 12. As shown in a second image 12 taken from a second viewing position P2 (with the first pattern 10 being oriented towards said second view 10 ing position P2) in a reflection viewing mode, the first region 11 appears in a darker color than the surrounding second region 12. A third image 13 taken from a third viewing position P3 (with the second pattern 20 being oriented is towards said third viewing position P3) in a reflection viewing mode shows the third region 23 in a lighter color than the surrounding fourth region 24. Thus, the third region 23 is discernible from the fourth region 24. 20 Figure 3 shows a security document 100 com prising a security device 1 according to a second embodi ment of the invention. The security device 1 is very sim ilar to the first embodiment shown in figure 2 with the exception that the first pattern 10 and the second pat 25 tern 20 are inverted grayscale images each comprising a plurality of pixels and not only two distinct regions. Thus, the first and second regions 11,12... (and likewise the third and fourth regions 23,24,...) are in general de fined by a single pixel each and not any more by geomet 30 rical letters/numbers. Other than that, as it is schematically shown on the right hand side of figure 3, the security device 1 according to the second embodiment behaves very much like the first embodiment discussed above, i.e., the different 35 regions /pixels in one pattern/image are indiscernible in a transmission viewing mode (first image Il from a first viewing position P1), while they are discernible in a re- WO 2015/095977 PCT/CH2014/000178 19 flection viewing mode (second image 12 from a second viewing position P2 with the first pattern 10 being ori ented towards the second viewing position and third image 13 from a third viewing position P3 with the second pat 5 tern 20 being oriented towards the third viewing position P3). Note that in this embodiment, as in the first embodiment shown in figure 2, the first pattern 10 is in verted with respect to the second pattern 20. Here, addi 10 tionally, care should be taken that grayscale values of the first and second patterns 10, 20 (x-values in a his togram, see ranges in figure 1 at the top!) only cover a range of black levels that lead to indiscernible result ing black level differences in transmission viewing mode is (see resulting black level differences on the y-axis of the diagram of figure 1). In other words, as an example, here, only gray levels between 35% black and 65% black are covered by the patterns 10,20, thus leading to super posed black levels (in the first image Il) between 77.25% 20 and 75% (see above). As discussed above, this is not dis cernible by a viewer's naked eye. Fig. 4 schematically shows a security device 1 according to a third embodiment of the invention. The 25 security device 1 is very similar to the first embodiment shown in figure 2 and to the second embodiment shown in figure 3 with the exception that the security device 1 additionally to the first pattern 10 (dark "OFS"=first region 11, medium background=second region 12, light 30 "123"=additional region of the first pattern 10) on the first surface 3 of the substrate 2 (which is not shown here for clarity) and to the second pattern 20 (inverted with respect to the first pattern 10, i.e., light "OFS"=third region 23, medium background=fourth region 35 24, dark "123"=additional region of the second pattern 20) on the second surface 4 of the substrate 2 comprises a third pattern 30 that is mixed into the first pattern WO 2015/095977 PCT/CH2014/000178 20 10 and into the second pattern 20 before the actual ap plication of the patterns. Another option which is not shown here would be to apply, in particular print, the third pattern 30 between single layers of the multi s layered substrate. The third pattern 30 comprises a fifth region 35, a sixth non-overlapping region 36, ... which are single pixels each. Then, as it is shown in the first image I1, in a transmission viewing mode, only the third pattern 30 10 is visible because the first pattern 10 and the second pattern 20 cancel each other out as discussed above with regard to the first two embodiments of the invention. However, in a reflection viewing mode as shown in second image 12 (first pattern 10 is oriented 15 towards the second viewing position P2), both the first pattern 10 and the third pattern 30 are visible (i.e., the first region 11 is discernible from the second region 12 and, respectively, the fifth region 35 is discernible from the sixth region 36). 20 In a reflection viewing mode as shown in third image 13 (second pattern 20 is oriented towards the third viewing position P3), both the second pattern 20 and the third pattern 30 are visible (i.e., the third re gion 23 is discernible from the fourth region 24 and, re 25 spectively, the fifth region 35 is discernible from the sixth region 36). Fig. 5 schematically shows a security docu ment 100 (a banknote with a denomination 501) comprising 30 the security device 1 of fig. 2. The security device 1 is arranged in a window of the security document 100 and a light absorber 5 consisting of a region with 100% black is arranged at a distance to the security device 1. If the security document 100 is folded along a folding line 3s 500, the light absorber 5 can be brought into overlap with the security device 1 and thus a reflection viewing mode is easier to achieve (see below).

WO 2015/095977 PCT/CH2014/000178 21 Fig. 6 schematically shows the security de vice 1 of fig. 2 in a transmission viewing mode. The se curity device 1 comprises the transparent substrate 2 s with the first surface 3 and the second surface 4. The first pattern 10 with the first region 11 and the second region 12 is arranged on the first surface 3 (only sche matically shown). The second pattern 20 with the third region 23 and the fourth region 24 is arranged on the io second surface 4 (only schematically shown). In a trans mission viewing mode (image Il at a viewer's first view ing position P1), for at least one transmitted wavelength through said security device, said first region is indis cernible from said second region and said third region is is indiscernible from said fourth region (only schematically shown). Fig. 7 schematically shows the security de vice 1 of fig. 2 in a reflection viewing mode with specu 20 lar reflection. In a reflection viewing mode (image 12 at a viewer's second viewing position P2), for at least one (specularly by the first surface 3) reflected wavelength from said security device, said first region is discerni ble from said second region (only schematically shown). 25 Fig. 8 schematically shows the security de vice 1 of fig. 2 in a reflection viewing mode with specu lar reflection and second pattern attenuation which is facilitated by a light absorber 5. The situation is es 30 sentially the same as in fig. 7, but in addition to only specular reflection on the first surface 3, a light ab sorber 5 is arranged at the second surface 4 and helps to attenuate the second pattern 20. This is due to the prop agation of light and the multiple reflections of the 3s light inside the substrate 2.

WO 2015/095977 PCT/CH2014/000178 22 Yet another embodiment is shown in Fig. 9. It shows a security device in the transmission viewing mode. The first pattern 10 is printed on the first surface 3 and the second pattern 20 is printed on the second sur 5 face 4. The first and second patterns e.g. comprise black levels in a range between 20% and 80% according to the color gradients shown in Fig. 9. The black levels on the first pattern are inverted with respect to the black lev els on the second pattern. The thicknesses of the arrows 10 show the light intensity in different steps on the way the light goes through the security device. The light, originating from a light source, goes uniformly through the second surface 4 and enters the substrate 2. Light is absorbed by the second pattern 20 in dependence of the 15 black levels of the second pattern. Consequently the light intensity going through the substrate 2, which is shown by the thickness of the arrows in the substrate 2, is the smaller the higher the black level is on the sec ond pattern 20. Since the black level on the first pat 20 tern 10 is inverted with respect to the second pattern 20, the light intensity is homogenized after having left the substrate 2 and having passed the first surface 3. Fig. 10 shows the same security device as in Fig. 9, but 25 in the reflection viewing mode. The substrate can reflect the light specularly or diffusely. The arrows show that the lower the black level on the first surface 3 is, the more light is reflected, because the black ink absorbs the light and does not reflect it. The effect of the sec 30 ond surface 4 is attenuated by the light absorber 5. Remark: While there are shown and described presently preferred embodiments of the invention, it is to be dis tinctly understood that the invention is not limited 3s thereto but may be otherwise variously embodied and prac ticed within the scope of the following claims.

Claims

1. A security device (1) for verifying an au thenticity of a security document (100), in particular of s a banknote, a passport, a document of value, a certifi cate, or a credit card, the security device (1) compris ing - an at least partially transparent substrate (2) and with a first surface (3) and a second surface io (4), - a first pattern (10) arranged on the first surface (3) of the substrate (2), - a second pattern (20) arranged on the sec ond surface (4) of the substrate (2), 15 wherein the first pattern (10) and the second pattern (20) each comprise a plurality of pixels with at least three different gray levels visible from a macro scopic perspective, wherein the first pattern (10) and the second 20 pattern (20) cover only gray levels in a range between 20% black and 80% black, in particular between 35% black and 65% black, wherein the first pattern (10) is inverted with respect to the second pattern (20). 25

2. The security device (1) of claim 1 wherein the first pattern (10) and the second pattern (20) are applied, in particular printed, by absorbing inks. 30

3. The security device (1) of any of the pre ceding claims wherein said substrate (2) comprises multi ple layers. 35 WO 2015/095977 PCT/CH2014/000178 24

4. The security device (1) of any of the pre ceding claims, wherein the gray levels of the first pat tern (10) and of the second pattern (20) are indiscerni ble at least when an overall transmitted light intensity s through said security device (1) outshines an overall re flected light intensity from said security device (1) at least by a factor of 5.

5. The security device (1) of any of the pre 10 ceding claims, wherein the gray levels of the first pat tern (10) and of the second pattern (20) are discernible at least when an overall reflected light intensity from said security device (1) outshines an overall transmitted light intensity through said security device (1) at least is by a factor of 5.

6. The security device (1) of any of the pre ceding claims further comprising a third pattern (30) ar ranged on or in said substrate (2), wherein said third 20 pattern comprises a plurality of pixels visible from a macroscopic perspective, wherein the different gray lev els of said third pattern (30) are discernible in said transmission viewing mode and in said reflection viewing mode. 25

7. The security device (1) of claim 6 wherein said third pattern comprises a plurality of pixels with at least three different gray levels visible from a mac roscopic perspective. 30 35

8. The security device (1) of any of the pre ceding claims wherein said first pattern (10) and/or said WO 2015/095977 PCT/CH2014/000178 25 second pattern (20) and/or said substrate (2) comprise a color filter.

9. The security device (1) of any of the pre s ceding claims wherein a transmittance of said substrate (2) is higher than 50%, at least for said at least one transmitted wavelength through said security device (1).

10. The security device (1) of any of the 10 preceding claims wherein a thickness of said substrate is smaller than 500 pm, in particular smaller than 120 pm.

11. A security document (100), in particular a banknote, a passport, a document of value, a certifi is cate, or a credit card, wherein the security document (100) comprises a security device (1) of any of the preceding claims, in particular arranged in a window of said security document (100). 20

12. The security document (100) of claim 11 further comprising a light absorber (5), in particular arranged at a distance to said security device (1). 25

13. The security document (100) of claim 12 wherein said light absorber (5) has a reflectivity of less than 50% and/or a transmittance of less than 50%.

14. A method for verifying an authenticity of 30 a security document (100) of any of the claims 11 to 13, the method comprising steps of - providing said security document (100) com prising a security device (1) of any of the claims 1 to 10, 3s - from a first viewing position (P1) acquir ing a first image (I1) of said security device (1) in a transmission viewing mode, WO 2015/095977 PCT/CH2014/000178 26 - from a second viewing position (P2) acquir ing a second image (12) of said security device (1) in a reflection viewing mode with said first pattern (10) be ing oriented towards said second viewing position (P2), 5 - deriving said authenticity of said security document (100) using said first image (II) and using said second image (12).

15. The method of claim 14 wherein said first io viewing position (P1) and said second viewing position (P2) are the same.

16. The method of any of the claims 14 to 15 wherein during said step of acquiring said second image is (12) of said security device (1), an overall reflected light intensity from said security device (1) outshines an overall transmitted light intensity through said secu rity device (1) at least by a factor of 5. 20

17. The method of any of the claims 14 to 16 wherein during said step of acquiring said first image (Il) of said security device (1), an overall transmitted light intensity through said security device (1) out shines an overall reflected light intensity from said se 25 curity device (1) at least by a factor of 5.

18. The method of any of the claims 14 to 17 comprising a further step of - bringing a light absorbing device (5) into 30 an overlap with said security device (1), wherein said step of acquiring said second image (12) of said security device (1) is carried out with said light absorbing device (5) being in said over lap with said security device (1). 35

19. The method of any of the claims 14 to 18 comprising a further step of WO 2015/095977 PCT/CH2014/000178 27 - from a third viewing position (P3) acquir ing a third image (13) of said security device (1) in a reflection viewing mode with said second pattern (20) be ing oriented towards said third viewing position (P3), 5 wherein said third image (13) is used in said step of deriving said authenticity of said security docu ment (100).

20. A method for generating a security device io (1) for verifying an authenticity of a security document (100), in particular of a banknote, a passport, a docu ment of value, a certificate, or a credit card, the meth od comprising steps of - providing an at least partially transparent 15 substrate (2) and with a first surface (3) and a second surface (4), - providing a first source image with at least three different gray levels, - modifying a contrast of said first source 20 image to generate a first modified image, wherein the first modified image covers only gray levels in a range between 20% black and 80% black, in particular 35% black and 65% black, - inverting the first modified image for 25 yielding a second modified image, - generating a first pattern (10) from said first modified image, - generating a second pattern (20) from said second modified image, 30 - applying, in particular printing, the first pattern (10) on said first surface (3) of said substrate (2), - applying, in particular printing, the sec ond pattern (20) on said second surface (4) of said sub 35 strate (2). WO 2015/095977 PCT/CH2014/000178 28

21. The method of claim 20 wherein, for gen erating the first pattern (10) and/or second pattern (20), a third pattern is mixed with the first modified image to generate the first pattern and/or wherein the 5 third source image is mixed with the second modified im age to generate the second pattern (20) such that in a transmission viewing mode only the third pattern (30) is visible while the first pattern (10) and the second pat tern (20) cancel each other out. 10

22. The method of any of the claims 20 to 21 wherein the first pattern (10) and the second pattern (20) are printed by absorbing inks. 15

23. The device, document or method of any of the preceding claims wherein said substrate is partially reflecting, in particular specularly reflecting.